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Jon Udell's LibraryLookup is the coolest.
After you've "installed" your bookmarklet in this way, you can look up books at your local library. Let's say you're on a book-related site (Amazon, BN, isbn.nu, All Consuming, possibly others), and a book's info page is your current page. (Specifically: its URL contains an ISBN. Choose a hardcover edition for best results -- see tips below.) You can click your bookmarklet to check if the book is available in your local library. The bookmarklet will invoke your library's lookup service, feed it the ISBN, and pop up a new window with the result.